Overlooked Secondary Effects in Open-Deck Truss Bridges

Deformation-induced fatigue cracking has been a common source of fatigue damage in steel bridges. These damage cases are often the result of secondary restraint forces generated by an unintended or overlooked interaction between different members in the bridge. A number of these interaction mechanisms are examined in this paper using finite element analysis of a case-study bridge, the old Forsmo Bridge in northern Sweden. Field measurements performed on the bridge were used to validate the analysis. The results of the analysis indicate that the effects of secondary forces, which develop in different members of the floor system of riveted truss bridges, are generally not negligible. In particular, the connections between different orthogonal members of the floor system, such as stringer-to-floor-beam and floor-beam-to-chord connections, might be critical with respect to fatigue due to these effects.